1. Field of the Invention
The invention is directed to a circuit and an associated method for potential-isolated energy transfer from a primary side to a secondary side of the circuit, wherein two output DC voltages having in each case different absolute values are intended to be present. These two output DC voltages can have identical or differing polarity.
2. Description of the Related Art
Circuits of this type are, for example, part of drive circuits for power semiconductor modules. One known basic circuit of a power semiconductor module is the half-bridge circuit of two power transistors, in which at least one of the two power transistors is not at a constant reference potential. Its reference potential can fluctuate on the order of magnitude of one kilovolt. Therefore, it is generally known to design the associated drive circuits with a primary side and at least one secondary side electrically isolated therefrom. In this case, the primary side has at least the task of forwarding the control pulses to the respective secondary side, and also of power transfer, for supplying energy to the secondary side.
To accomplish this power transfer, transformers are frequently used and well known. In their simplest configuration, transformers are embodied with a primary-side and a secondary-side winding and fed by an AC voltage source. On the secondary side, this AC voltage, in order to generate a DC voltage such as is necessary for example for driving the gate of a power transistor, is rectified and, if appropriate, filtered.
In a large number of power transistors, it is preferred, as is known, to switch them on with a positive voltage and to switch them off with a negative voltage. In this case, for energy-related reasons, it can be advantageous to choose the absolute value of the switch-off voltage to be less than the value of the switch-on voltage as defined by the design of the power transistor.